During the last few years, cellular operators have started to offer mobile broadband based on Wideband Code Division Multiple Access (WCDMA)/High Speed Packet Access (HSPA). Further, fuelled by Long Term Evolution (LTE)/LTE Advanced (LTE-A) deployments and new devices designed for data applications, end user performance requirements are steadily increasing. The large uptake of mobile broadband has resulted in significant growth in the traffic volume that needs to be handled by the HSPA/LTE/LTE-A networks. Therefore, techniques that allow cellular operators to manage their networks more efficiently are of large importance. One such technique is deployment of Low Power Nodes (LPNs) in a coverage area of a macro base station, which may also be referred to herein as a macro node or a high power node. This type of deployment is referred to as a heterogeneous network.
A homogeneous network is a network of base stations (e.g., Node Bs NBs)/enhanced or evolved NBs (eNBs)) in a planned layout and a collection of User Equipment devices (UEs), which may also be referred to herein as user terminals, in which all base stations have similar transmit power levels (typically 43-46 Decibel-milliwatts (dBm)), antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted access to UEs in the network, and serve roughly the same number of UEs. Many current deployments of wireless systems fall under this category. For example, many current deployments of Global System for Mobile communications (GSM) networks, WCDMA, High Speed Downlink Packet Access (HSDPA) networks, LTE networks, and WiMax networks are homogeneous networks.
In heterogeneous networks, in addition to the planned or regular placement of macro base stations (referred to as the macro layer), several micro/pico/femto/relay/Remote Radio Unit (RRU) nodes (commonly referred to as LPNs) are deployed. One example of a heterogeneous network 10 is illustrated in FIG. 1. As shown, the heterogeneous network 10 includes a macro base station 12, which may be referred to as a High Power Node (HPN) or macro node, and a number of LPNs 14 within the coverage area of the macro base station 12. Note that the power transmitted by the LPNs 14 is relatively small compared to that of macro base station 12, e.g. 2 Watts (W) as compared to 40 W for a typical macro base station. The LPNs 14 are deployed to eliminate coverage holes in the homogeneous network (i.e., the network using macro base stations only) and to off-load the macro layer, thereby improving the capacity in hot-spot scenarios. Due to the lower transmit power and smaller physical size, a LPN 14 might offer more flexible site acquisitions.
Generally, there are two different types of deployments of a heterogeneous network 10. In a first type of deployment, which is referred to as a co-channel deployment, each LPN 14 has its own cell identity (scrambling code in WCDMA/HSPA), and the LPNs 14 and the macro base station 12 provide different cells (i.e., small cells provided by the LPNs 14 are different than the macro cell provided by the macro base station 12) but the cells typically share the same frequency. In the second type of deployment, which is referred to as a soft or combined cell deployment, the LPNs 14 and the macro base station 12 operate together to provide the same cell (i.e., the small cells provided by the LPNs 14 have the same cell identity as the macro cell provided by the macro base station 12).
FIG. 2 shows one example of co-channel deployment of the heterogeneous network 10. In this example, the macro base station 12 creates a macro cell (cell A), and two LPNs 14 create small cells B and C, respectively. Each individual cell is characterized by individual pilot signals, downlink and uplink control channels, and data traffic channels.
FIG. 3 shows one example of a soft or combined cell deployment of the heterogeneous network 10. In this example, the LPNs 14 are part of the macro cell. This setup avoids the frequent (soft) handovers, and hence higher layer signaling. Note that in this deployment all the nodes are coupled to the central node (in this case the macro node) via a high speed data link.
FIG. 4 shows the average sector throughput in megabits per second (Mbps) versus number of UEs per macro node with four LPNs with 37 dBm and 30 dBm power for WCDMA. It can be seen that, at high load, the co-channel deployment gives significant gains because more users are offloaded. This is referred to herein as gains due to load balancing.
FIG. 5 shows the percentage of gain (with respect to a homogeneous network) achieved with a co-channel deployment of a heterogeneous network. It can be observed that, at low loads, there is almost no gain and that the gain increases as the load increases. The gain depends on the percentage of offloading.
Since the LPNs 14 have less transmit power and hence a smaller coverage area than the macro base station 12, the number of UEs served by the LPNs 14 are less compared to that of the macro base station 12. The gains in the heterogeneous network 10 can be improved if more UEs are offloaded to the LPNs 14. One technique to improve the overall system throughput is cell range expansion where the UEs are offloaded to the LPNs 14 by increasing the Cell Individual Offsets (CIOs) used for handover decisions.
FIG. 6 illustrates one example of cell range expansion. As illustrated, cell range expansion results in a cell range expansion zone 16 around the cell of the LPN 14. In the cell range expansion zone 16, the strongest cell is the macro cell. However, if the LPN 14 is less loaded than the macro cell, UEs within the cell range expansion zone 16 can be served more often by the LPN 14, even though the throughput may be reduced due to the small cell of the LPN 14 not being the strongest cell. Since these UEs get scheduled more often when connected to the LPN 14, the overall throughput is higher.
When in the cell range expansion zone 16, a UE that is connected to the LPN 14 experiences strong interference from the macro base station 12. FIG. 7 shows the link throughput when the UE is connected to the LPN 14 with different macro interference values (Ioc). It can be observed that the performance of the UE is severely impacted when the dominant interferer power is 10 to 20 times that of received power from the LPN 14. It can be observed from above that there is huge performance degradation with the interference. The performance loss is in the range of 100% at high geometries. Hence, even though the UE is offloaded to the LPN 14 when in the cell range expansion zone 16, the individual UE throughput is impacted in the cell range expansion zone 16.
As such, there is a need for systems and methods for improving performance of user terminals in the cell range expansion zone of a LPN in a heterogeneous network.